Bad news for the fans of lunar exploration: last week, the ESA ministerial council decided to not to allocate further funding for the Lunar Lander, giving higher priorities for other areas, like the ExoMars mission. Not all hope is lost though: DLR, the German space agency vowed to continue the work on the project and is looking for partners, but even if they succeed, the mission will slip into the 2020s. In the mean time, however, a more radical proposal is making its way to the leaders of the agency: let's smash a giant space telescope into the Moon!

The idea is less crazy as it first sounds. The grandiose Herschel space telescope of ESA, equipped with a primary mirror that dwarfs even Hubble's, is nearing its end, and nothing can be done about that. Herschel, being an infrared telescope, has to be cooled, otherwise it would itself glow in the wavelengths it's supposed to detect. 2400 liters of 1.7 K cold superfluid helium and some clever cooling methods keep the detectors at only 0.3 K. Unlike in an ordinary refrigerator, however, this coolant is slowly leaking away. Here, on the Earth, the flow of ever-present air carries the heat away from the condenser (the big black metallic grid) on the back of the fridge. In the vacuum of space, heat can be disposed of only with radiation – but that wouldn't be wise, as we want to limit all excess infrared light in the vicinity of the telescope. (And probably wouldn't be sufficient either, but I haven't done the math.) So, until some reliable active, mechanical cooling system finds its way to space, the only means of cooling is to let the helium itself slowly boiling away, carrying away the excess heat. Thus, except for the shortest wavelengths, the amount of helium and the efficiency of the cooling system gives a hard limit for the scientific lifetime of any infrared telescope.

The cryostat of Herschel: the detectors and the helium tank (left) and the entire assembly, with the dispensed gaseous helium (right)

The helium aboard Herschel will run out sometime in March 2013. The space telescope is currently orbiting the L2 Earth-Sun Lagrangian point, about 1.5 million kms away from the Earth, but that orbit is unstable and has to be maintained. Herschel will have to be removed from L2, but its fate is not yet decided. Two options are currently under investigation: one is to simply send the spacecraft away into a heliocentric orbit, where it will not encounter the Earth again for a few centuries. The spacecraft wouldn't be shut down right away: Herschel carries a radiation monitor that is continuously observing the rate of energetic particles in the Solar System. Gathering data beyond the imminent solar maximum or even through a full solar cycle would be splendid, but it would also require to keep the spacecraft alive for a few more years.

Proton measurements of the radiation detector aboard Herschel showing two solar storms during January 2012. Energy levels increase from the top towards the bottom.

The other proposal is to navigate the spacecraft back towards the Earth-Moon system and let it impact one of the polar craters of the Moon. (A third proposal existed a few months ago, testing spacecraft operations in the Earth-Moon L2 point, but that would eventually end with a lunar impact anyway.) The dry mass of the spacecraft is 2.8 metric tons, slightly more than the Centaur rocket stage used in the LCROSS mission. The impact would create a similar crater, about 30 m wide and 5 m deep and excavate the same magnitude of material from the surface. The target, however, might not be the bottom of another permanently shadowed polar crater. According to our current knowledge, not only the deep interiors of polar craters hide water ice, but the partially exposed rims might also hide various volatiles, shielded by a thin layer of regolith. These areas would be more accessible to future human explorers, but the hypothesis has to be experimentally confirmed. Lacking any shepherding spacecraft, the impact location should also be tailored for other means of observations: ground-based telescopes, Hubble and the Lunar Reconnaissance Orbiter will be available.

Elevation map (left) and radar detection of potential water ice content (right) of Shackleton crater at the lunar South Pole. Measurements indicate that not only the very ragged, permanently shadowed bottom but also the smooth, partially lit, much more accessible walls of the crater contain volatiles. That's reassuring for future human explorers. Measurements were made with the laser altimeter and the Mini-RF radar onboard LRO.

Whichever scenario will happen, neither happen fast: the thrusters of Herschel aren't meant for deep-space travel. Escape into heliocentric orbit can be achieved only in favorable orbital conditions, in May or October 2013. If Herschel heads towards the Moon, June or July 2013 is the earliest to reach it. Some, of course, strongly resist the idea of smashing their most beloved asset to smithereens so unceremoniously, but sentiment and the cold reality of spaceflight don't always fit together.

The Solar System itself litters the Moon: this small, ~10 m wide crater with bright, extended rays was created when a lump of rock with a diameter of only ~0.5 m hit the surface some time between 1971 and 2009.

And don't worry about littering the Moon, either: at the tremendous speed of the impact (about 2.8 km/s), all the material consisting of Herschel, detectors, mirrors, sunshields, fuel, will be vaporized instantly, and will dissolve into the gas and dust kicked up by the impact – just like in the case of the natural impacts, numbers beyond counting, that have been (and will be) littering the Moon for billions of years.

László Molnár

Image sources and credits:

1.) NASA
2.) ESA
3.) ESA/AOES Medialab
4.) ESA/ESTEC
5.) NASA/Zuber et al. Nature, 2012
6.) NASA/GSFC/Arizona State University